Ericsson have both their own PKI infrastructure[0], at least for software integrity checking (however that certificate is valid since March this year, and the CRL[1] it refers to is empty), in addition to using other certificate authorities for everything such as the hosting of websites to internal infrastructure[2]. I suspect it wasn't any of the above - rather it is the PKI that is used in running the IPSec networking done between carrier's RANs and other parts of core network[3], which is probably Ericsson's own internal CA.

[0] https://www.ericsson.com/en/about-us/enterprise-security/pki

[1] http://crl.ericsson.net/Ericsson_Software_Deliverable_Integr...

[2] https://crt.sh/?q=%.ericsson.net

[3] https://en.wikipedia.org/wiki/System_Architecture_Evolution